Photooxidation of Methacrolein in Fe(III)-Oxalate Aqueous System and Its Atmospheric Implication

Yu WANG, Jie ZHAO, Huihui LIU, Yuan LI, Wenbo DONG*,2, and Yanlin WU*,2

1Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention, Department of Environmental Science& Engineering, Fudan University, Shanghai 200438, China

2Shanghai Institute of Pollution Control And Ecological Security, Shanghai 200092, China

ABSTRACT Iron and oxalic acids are widely distributed in the atmosphere and easily form ferric oxalate complex (Fe(III)-Ox). The tropospheric aqueous-phase could provide a medium to enable the photo-Fenton reaction with Fe(III)-Ox under solar irradiation. Although the photolysis mechanisms of Fe(III)-Ox have been investigated extensively, information about the oxidation of volatile organic compounds (VOC), specifically the potential for Secondary Organic Aerosol (SOA) formation in the Fe(III)-Ox system, is lacking. In this study, a ubiquitous VOC methacrolein (MACR) is chosen as a model VOC, and the oxidation of MACR with Fe(III)-Ox is investigated under typical atmospheric water conditions. The effects of oxalate concentration, Fe(III) concentration, MACR concentration, and pH on the oxidation of MACR are studied in detail. Results show that the oxidation rate of MACR greatly accelerates in the presence of oxalate when compared with only Fe(III). The oxidation rate of MACR also accelerates with increasing concentration of oxalate. The effect of Fe(III) is found to be more complicated. The oxidation rate of MACR first increases and then decreases with increasing Fe(III) concentration. The oxidation rate of MACR increases monotonically with decreasing pH in the common atmospheric water pH range or with decreasing MACR concentration. The production of ferrous and hydrogen peroxide, pH, and aqueous absorbance are monitored throughout the reaction process. The quenching experiments verify that ·OH andare both responsible for the oxidation of MACR. MACR is found to rapidly oxidize into small organic acids with higher boiling points and oligomers with higher molecular weight, which contributes to the yield of SOA. These results suggest that Fe(III)-Ox plays an important role in atmospheric oxidation.

Key words: Fe(III)-Ox, OH radical, atmospheric oxidation, SOA, methacrolein

1. Introduction

Tropospheric aqueous-phase chemistry plays a key role in the formation of oxidizing species in the atmosphere (Herrmann et al., 2015). Iron is the most abundant transition metal element in tropospheric particles. It enters the atmosphere through various ways, such as sea spray (Guasco et al., 2014), combustion and industrial activities, and from the earth via wind (Liati et al., 2013; Li et al., 2016). Fenton reaction caused by the presence of iron is widespread in the atmosphere (Deguillaume et al., 2004). Some studies (Nguyen et al., 2013; Chu et al., 2014) have indicated that the Fenton reaction in the atmosphere mainly happens in the aqueous phase. Some estimates suggest that Fenton reaction involving iron might account for ～30% of theOH production in fog droplets (Deguillaume et al., 2005). The oxidizing ability contributed from Fenton reaction is highly dependent on pH, iron concentration, and the kind and concentration of organic compounds that form complexes with iron(Zuo and Hoigne, 1992; Weller et al., 2013b, a). Notably,the formation of iron complexes have a significant effect on the photochemical reaction pathway. Iron can complex with oxalic acid, which was the most abundant dicarboxylic acid in aqueous aerosol (Sorooshian et al., 2006; Legrand et al.,2007), and the ferric oxalate complex (Fe(III)-Ox) can efficiently produced oxidized species by photo-reduce Fe(II)under relevant atmospheric condition (Thomas et al., 2016).Oxalic acid is an ultimate end product in the oxidation of organic compounds in atmosphere such as isoprene and aromatic hydrocarbons (Boreddy et al., 2017). Oxalic acid could also come from incomplete combustion. One simulation has shown that at least 50% of Fe(III) is bound by oxalic acid during both cloud and deliquescent particle periods because of their strong complexing coefficients (Weller et al., 2014), and 99% of oxalic acid is consumed by photolysis. Field observations have also shown a significant negative correlation between atmospheric oxalic and Fe(III) concentrations (Sorooshian et al., 2013). Photolysis of Fe(III)-Ox in the atmosphere is extensive. Thus, it is necessary to evaluate its effect on atmospheric oxidation. Although the photochemistry of Fe(III)-Ox has been studied extensively,knowledge of its impact on the aging of atmospheric organic matter, especially a volatile organic compound(VOC) with a low boiling point in the aqueous phase, is still inadequate (Thomas et al., 2016; Pang et al., 2019). There is a potential for secondary aerosol formation when a VOC with a low boiling point transfers to a product with a higher boiling point.

Isoprene (2-methyl-1,3-butadiene, CH) is a widespread biogenic hydrocarbon (Seinfeld and Pandis, 2016)that relates closely to photosynthetic activity in plants. Its main oxidation product is methacrolein (CHO, MACR),which occupies about 22% of the yield of isoprene. MACR is a model compound in photochemical experiments.Although MACR has a small Henry coefficient(H=6.5 M atm), its concentration in the aqueous phase is found to be much higher than expected based only on its Henry coefficient (van Pinxteren et al., 2005). Thus,MACR is a typical representative VOC in the aqueous phase. The oxidation mechanism of MACR has been investigated previously in many studies (Zhang et al., 2010; Liu et al., 2012; Schöne et al., 2014; Giorio et al., 2017), allowing for easy comparisons.

This study explores the aqueous photo-Fenton reaction involved in the oxidation of MACR in the presence of Fe(III)-Ox. The effects of oxalate concentration, Fe(III) concentration, MACR concentration, and pH are analyzed. The main long-lived oxidation species, Fe(III) and HO, in photo-Fenton reaction are measured. The kinds and abilities of free radicals involved in the reaction with MACR are evaluated. The photodegradation mechanism and the possible oxidative products are studied by liquid-chromatography quadrupole time-of-flight mass spectrometry (LCQTOF-MS). The oxidation products (namely, small organic acids) are analyzed by ion chromatograph (IC). Furthermore, the absorbance of the aqueous solution is tracked by a UV-vis spectrometer. The results obtained in this study will provide a reference for predicting the influence of low boiling-point VOCs in photooxidation with Fe(III)-carboxylic complexes in atmospheric water.

2. Materials and methods

2.1. Materials

MACR (95%) and potassium oxalate (99.98%) were purchased from Shanghai Aladdin Bio-Chem Technology Co.Ltd. Ferric perchlorate, perchloric acid (70%), ferrozine(97%), 4-hydroxyphenylacetic acid (99%), and peroxidase(from horseradish, RZ＞2.5) were purchased from Sigma-Aldrich. Analytical grade CHCOOH, CHCOONa, NaOH,hydroxylamine hydrochloride, phenazine, and ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) were purchased from Sinopharm Chemical Reagent Co., Ltd. Deionized (DI) water was utilized in all experiments.

2.2. Photolysis experiments

Photolysis reaction was performed in a photochemical reactor (XPA-VII, Xujiang electromechanical plant,Nanjing, China) equipped with a 500 W Xenon lamp combined with a 290 nm cut-off filter as simulated solar light.The Xenon lamp spectrum received by the reaction solution is shown in Fig. A1 in the Appendix. The temperature of the reactor was maintained at 25°C ± 2°C by cooling water.The pH was adjusted by HClO. A 20 mL solution was prepared under constant magnetic stirring and reacted in a 50 mL cylindrical quartz tube sealed with a rubber stopper.Before sealing, the aqueous solution, which did not contain MACR, was stirred in the dark for 10 min to achieve oxygen saturation. Then, 1 mL of the solution was taken out through the rubber stopper using a syringe and filtered through a poly tetra fluoroethylene (PTFE) 0.22 μm filter.

2.3. Analytical procedures

GC-FID (Agilent 7890A, USA) was used to analyze the MACR concentration. It was equipped with a semi-polar capillary column (CNW CD-5MS 30 m × 0.25 mm × 0.25 μm, Germany), which allowed the injection of aqueous phase samples. The GC injector and detector were heated at 250°C. Nitrogen gas was used as carrier gas at 1 mL minwith a 1/10 split. The oven temperature program was 40°C for 4 min, raised by 10°C minup to 120°C, and 120°C for 5 min (El Haddad et al., 2009; Liu et al., 2009).

An IC (DIONEX ICS-3000, Thermo Fisher, USA)assembled with AG11-HC guard column and AS11-HC analytical column was used to measure organic acid anions(Zhou et al., 2018). The pH of the aqueous solution containing iron ion was adjusted to 10. Then, the solution was passed through a 0.22 μm PTFE filter and an H-type pretreatment filter to remove the iron.

The concentrations of Fe(II) and total dissolved iron were determined by using a spectrophotometry methodology with ortho-phenanthroline and hydroxylamine (Sastry and Rao, 1996). Hydrogen peroxide concentrations were determined by the fluorescence method developed by Lazrus et al. (1985) after sample mixing with EDTA-2Na.Solution pH was measured using a pH meter (METTLER TOLEDO FE20, Switzerland). UV-vis spectra of the solutions were tracked by UV-vis spectrometer (Scinco Nano MD, SCINCO, South Korea).

Reaction byproducts were identified with a LC-QTOFMS system (Agilent 6540, USA) equipped with an electrospray ionization source, and analysis was performed under negative ionization mode (Lian et al., 2017).

3. Results and discussion

3.1. The control experiments

As seen in Fig. 1, MACR shows little change under dark conditions. The small Henry coefficient of MACR keeps it mostly distributed in the aqueous solution, and errors caused by gas-liquid distribution are ignored in subsequent experiments. No obvious direct photolysis of MACR is found under the simulated sunlight conditions (λ＞ 290 nm) because MACR’s absorbance peak at 375 nm is low (Fig. 2). In the presence of Fe(III), MACR is oxidized about 7% in 90 min. F e(OH)is the dominant Fe(III) hydroxide complex in the aqueous solution with pH of 4, and it is expected to undergo photolysis to generateOH according to Eqs. (1−3) and then react with MACR.

where h is Planck constant, v is frequency of light.

In the presence of Fe(III)-Ox (Fig. 1), the oxidation rate of MACR rapidly increases, and 80% MACR is oxidized in 60 min. This result occurs because Fe(III)-Ox forms and the efficiency of Fenton-like reaction is accelerated. As shown in Fig. A2, Fe(III)-Ox has more obvious absorption peaks in the sunlight range compared to the Fe(III) solution.Quantum yields (at 293 K, pH = 4.0) for the photolysis of Fe(OH)at 313 nm and 360 nm are 0.14 ± 0.04 and 0.017 ±0.003 (Faust and Hoigné, 1990), respectively. Fe(OH)has weak absorption in the solar UVA-visible region (Knight and Sylva, 1975). Quantum yields for [Fe(III)(CO)]at 313 nm and 366 nm are 2663 ± 37 and 709 ± 10, respectively, and quantum yields for [Fe(III)(CO)]at 313 nm and 366 nm are 2055 ± 111 and 1.17 ± 1.46 (Weller et al.,2013b), respectively, which are much higher than Fe(OH), indicating that higher photoactivity produces more radicals.

3.2. Effect of oxalate concentration

The effect of oxalate concentration is investigated in the range from 100 μM to 2000 μM. As shown in Fig. 2a,the oxidation rate of MACR increases with increasing oxalate concentration. The pseudo-first-order oxidation rate of MACR has an increase of nearly equal proportion with increasing oxalate concentration from 100 μM to 2000 μM(Fig. 2b). Approximately 80% MACR is oxidized when oxalate concentration is 1000 μM. Oxalate at 2000 μM leads to the oxidation of all MACR, though this concentration is excessive. However, the oxidation rate corresponding to 2000 μM oxalate still increases linearly, and the inhibitory effect caused by excess oxalate is not observed in Fig. 2b.This result may be due to the low second-order reaction rate constant of oxalate andOH.

Fig. 1. Control experiments of MACR oxidation ([MACR]0 =500 μM, [Fe(III)]0 = 100 μM, [Oxalate]0 = 1000 μM, pH = 4.0± 0.1, air-saturated solution).

Fig. 2. (a) Effect of oxalate concentration on the photooxidation of MACR in the Fe(III)-Ox system; (b)The reaction rate constant calculated by the pseudo-first-order kinetics variation with the concentration of oxalate ([Oxalate]0 = 1000 μM, [MACR]0 = 500 μM, pH 4.0 ± 0.1, air saturated solution).

Fig. 3. Effect of Fe(III) concentration on the photooxidation of MACR in the Fe(III)-Ox system([Oxalate]0 = 1000 μM, [MACR]0 = 500 μM, pH 4.0 ± 0.1, air-saturated solution).

3.3. Effect of Fe(III) concentration

3.4. Effects of solution pH

3.5. Effects of MACR concentration

The influence of initial MACR concentration (from 60 μM to 680 μM) is investigated (Fig. 5). The oxidation rates of MACR increase with decreasing MACR concentration,and 60 μM, 160 μM, and 250 μM MACR are 100% oxidized during 45 min. However, a certain amount of MACR remains when MACR concentration is high (such as 400 μM and 680 μM). This is because oxalate is depleted and is unable to produce enoughOH to completely oxidize MACR.

Fig. 4. Effect of pH on the photooxidation of MACR in the Fe(III)-Ox system ([Fe(III)]0 = 100 μM,[Oxalate]0 = 1000 μM, [MACR]0= 500 μM, air-saturated solution).

Fig. 5. Effect of MACR concentration on the photooxidation of MACR in the Fe(III)-Ox system([Fe(III)]0 = 100 μM, [Oxalate]0 = 1000 μM, pH 4.0 ± 0.1, air-saturated solution).

3.6. Photooxidation mechanism of MACR in Fe(III)-Ox system

HOand Fe(II) are the major long-lived oxidized active species in the photolysis of Fe(III)-Ox (Faust et al.,1993), and this determines the production of free radicals.As shown in Fig. 6a, Fe(II) concentration rapidly increases to 80% in 10 min, and then reduces to approximately 2%from 30 min to 60 min. Finally, Fe(II) concentration is maintained at an undetectable value in the remaining time. The total dissolved iron remains stable for 30 min and then quickly drops to only 5 μM left in the next 30 min. Fe(II) is usually formed by the photoreduction of [Fe(III)(CO)]or [Fe(III)(CO)]and is finally consumed with HOto form Fe(III). The accumulation of Fe(II) is due to the fast photoreduction rate. The change of total dissolved iron happens because the iron precipitates as insoluble Fe(OH)(s),which is caused by decreasing oxalate concentration (Fig.A3) and increasing pH (Figs. 4. and A5). The change of pH is caused by the generation of OHin Fenton reaction.Fenton reaction can increase pH, as confirmed in a previous study (de Lima Perini et al., 2013). The pH increase can be attributed to the consumption of oxalate, leading to the release of Fe(III). The concentration of free oxalate changes, which might establish the acid-base reaction (de Luca et al., 2014). Otherwise, some organic acids are formed by the reaction of MACR andOH (Liu et al., 2009),which can decrease the solution pH.

Fig. 6. (a) The change of Fe(II), total dissolved iron, and pH (b) H2O2 formation during the photooxidation of MACR in the Fe(III)-Ox system ([Fe(III)]0 = 100 μM, [Oxalate]0 = 1000 μM, [MACR]0 = 500 μM, pH 4.0 ±0.1, air-saturated solution).

Fig. 7. Effect of scavengers on the photooxidation of MACR in the Fe(III)-Ox system ([Fe(III)]0 = 100 μM, [Oxalate]0 =1000 μM, [MACR]0 = 500 μM, pH 4.0 ± 0.1, air-saturated solution).

Fig. 8. Scheme for the proposed reaction pathways of the photooxidation of MACR in the Fe(III)-Ox system. The molecular formulas in the blue frames are observed products.

Fig. 9. Concentration of small organic acids in the photooxidation of MACR in H2O2 and Fe(III)-Ox system at 60 min obtained by IC ([Fe(III)]0 = 100 μM, [Oxalate]0 = 1000 μM, [MACR]0 = 500 μM, pH 4.0 ± 0.1).

As shown in Fig. 10, the absorbance of the solution changes significantly with irradiation time. Two big absorbance peaks are found at around 280 nm and 310 nm at the beginning of the reaction, which from oxalate and MACR,respectively. The absorbance at 310 nm decreases in the first 30 min, and the absorbance of 280 nm relative to the nearby baseline decreases in the first 10 min, increases from 10 min to 60 min, then decreases from 60 min to 180 min.The absorbance of the region with a wavelength higher than 320 nm increases from 30 min to 150 min. The increasing absorbance at 280 nm might be due to the

n

→πtransition of the intermediate products, such as aldehyde or ketone.The increasing absorbance of other parts comes from the formation of yellow precipitates Fe(OH), which absorb light in the short wavelength, not from the oxidation products of MACR (Fig. A6). The clear yellow precipitates are produced in the late stage of the reaction. This finding indicates that the absorbance change caused by Fenton reaction might be an ignored factor, which would change the optical properties of aerosols and has implications for radiative forcing (Pillar-Little and Guzman, 2018).

Fig. 10. The UV-Vis spectra of the photooxidation of MACR in Fe(III)-Ox system after filtering with 0.22 μm filter([Fe(III)]0 = 100 μM, [Oxalate]0 = 1000 μM, [MACR]0 = 500 μM, pH 4.0 ± 0.1, air-saturated solution).

4. Conclusions

Acknowledgements

. The authors gratefully acknowledge financial support from the Ministry of Science and Technology of the People’s Republic of China (Grant Nos. 2017YFC0210005 and 2016YFE0112200).

APPENDIX

Fig. A1. The xenon lamp spectrum received by the reaction solution.

Fig. A2. UV-vis absorption spectra of solution (pH = 4.0 ±0.1, in dark condition).

Fig. A3. Change of Fe(III) species distribution with oxalate concentration, [Fe(III)]0 = 100 μM, pH = 4.0, 25°C.

Fig. A4. Change of Fe(III) species distribution with Fe(III)concentration, [Oxalate]0 = 1000 μM, pH = 4.0, 25°C.

Fig. A5. Change of Fe(III) species distribution with pH,[Fe(III)]0 = 100 μM, [Oxalate]0 = 1000 μM, 25°C.

Fig. A6. The UV-Vis spectra of filtered (0.22 μm) and unfiltered (a) photolysis of the Fe(III)-Ox system, and(b) the photooxidation of MACR in the Fe(III)-Ox system ([Fe(III)]0 = 100 μM, [Oxalate]0 = 1000 μM,[MACR]0 = 500 μM, pH 4.0 ± 0.1, air-saturated solution).